Batteries include separators positioned between the anode and the cathode in order to prevent an electrical connection between the anode and the cathode, or a short-circuit. A short-circuit develops when conductive particulates bridge the separator or the separator deteriorates to the point where it allows the electrodes to touch. Rarely does a battery short-circuit occur all at once but rather over time by the building up of very small conductance paths termed “soft shorts.” “Dendrite shorting” refers to the situation where a dendrite comprising deposits, e.g., zincates in the case of alkaline batteries or lithium metal in the case of lithium batteries, form on one electrode of the battery and grow through the separator to the other electrode, resulting in an electrical connection between the anode and cathode.
Primary alkaline batteries generally have a cathode, an anode, a separator disposed between the cathode and the anode and an alkaline electrolyte solution. The cathode is typically formed of MnO2, carbon particles and a binder. The anode can be formed of a gel including zinc particles. The electrolyte solution, which is dispersed throughout the battery, is most commonly an aqueous solution containing 30–40% potassium hydroxide. Battery separators used in alkaline batteries have certain performance requirements. For instance, such separators need stability in the presence of strong alkaline electrolytes (e.g. 30–40% KOH). Lack of alkali chemical resistance can result in internal short circuiting between the electrodes due to loss of mechanical integrity. Good electrolyte absorption is also necessary, meaning the separator is sufficiently impregnated with the electrolyte solution necessary for the electrochemical reaction of the cell. Another requirement of the separator is to be a barrier to the growing dendrites of conductive zinc oxide formed by the electrochemical reaction in the cell, which can penetrate the separator and cause a short circuit. The separator also must allow the movement of ions between the electrodes, in other words the separator should exhibit low resistance to the flow of ions.
Secondary alkaline Zinc-MnO2 batteries have similar anodes, cathodes and electrolytes as primary alkaline batteries. Certain additives (e.g. Bi2O3, BaSO4, organic inhibitors, etc.) are often added to the anode and the cathode to improve the reversibility so that the battery can be recharged after having been discharged and to reduce zinc corrosion. During charge and discharge, some of the additives can dissolve into the electrolyte and migrate to the other electrode. The use of a separator with good dendritic barrier properties will help extend the cycle life of Zinc-MnO2 secondary batteries.
Battery separators for alkaline batteries are conventionally either thick, multi-layered nonwovens having large pores that have good (low) ionic resistance but relatively poor barrier to growing dendrites (also referred to herein as “dendritic barrier”), or multi-layered nonwovens with microporous membranes thereon having very small pores that have good dendritic barrier but very high ionic resistance. It would be desirable to have alkaline batteries with separators having improved balance of dendritic barrier and ionic resistance.
International Publication Number WO9953555 discloses a composite battery separator including at least one nonwoven layer and a layer that reduces dendrite shorting, which may be a microporous layer of cellophane, polyvinyl alcohol, polysulfone, grafted polypropylene or polyamide. The thickness of the composite separator is about 8.3 mils thick. The battery separator has an ionic resistance of less than about 90 mohms-cm2 when measured at 1 KHz in 40% potassium hydroxide (KOH) electrolyte solution. The microporous layer desirably has a very high level of barrier to air, but also undesirably a high ionic resistance, poor electrolyte wettability, and poor electrolyte absorption properties.
U.S. Pat. No. 4,746,586 discloses the use of PVA fibers having a denier of 0.8 or less to reduce the thickness and improve the barrier properties of battery separators for use in alkaline batteries, combined with cellulose fibers having a denier of no less than 1.0. If the cellulose fiber denier is reduced below this, the higher surface area fibers will lead to a higher rate of degradation.
Lithium batteries fall into three general categories, lithium primary batteries, lithium-ion secondary batteries and lithium-ion gel polymer batteries. Lithium primary batteries utilize many different types of battery chemistries, each using lithium as the anode, but differing in cathode materials and electrolytes. In the lithium manganese oxide or Li—MnO2 cell, lithium is used as the anode and MnO2 as the cathode material; the electrolyte contains lithium salts in a mixed organic solvent such as propylene carbonate and 1,2-dimethoxyethane. The lithium iron sulfide or Li/FeS2 battery uses lithium as the anode, iron disulfide as the cathode, and lithium iodide in an organic solvent blend as the electrolyte. Lithium-ion secondary batteries use lithiated carbon as the anode, lithium metal oxide (e.g. LiCoO2) as the cathode and a blend of organic solvents with 1 M lithium hexafluorophosphate (LiPF6) as the electrolyte. Lithium-ion gel polymer batteries use similar anode and cathode materials as lithium-ion secondary batteries. The liquid organic electrolyte forms a gel with the polymeric separator, which helps provide a good bond between the separator and the electrodes. The ionic resistance of the gel electrolyte is higher than that of liquid electrolytes but the gel electrolyte provides some advantages in terms of safety and form factor (i.e., the ability to form a battery into different shapes and sizes).
International Publication Number WO0189022 discloses a super fine fibrous porous polymer separator film for use as a battery separator in a lithium secondary battery, the separator film having a thickness of 1 μm to 100 μm. The separator film is formed from fine fibers having a diameter of between 1 and 3000 nm made by electrospinning a polymeric melt or polymeric solution.
In recent years, due to the miniaturization of electronic equipment, batteries must be made smaller without sacrificing the performance of conventional batteries. Nonwoven materials conventionally used as separators in alkaline batteries have large diameter fibers, thus making it difficult to achieve thin separators. Such nonwovens also have large pores, e.g. between about 15 μm and about 35 μm. The particles of the anode and the cathode may migrate towards each other through the large pores to cause an internal short circuit. In order to compensate for the large pore size and improve the dendritic barrier of the separators, i.e., protection from short-circuiting, thicker separators are made by using multiple layers. The thicker separators result in higher ionic resistance which is undesirable from a battery performance point of view. Moreover, these types of thicker separators cannot be used in certain designs, especially coin cells and other small battery designs useful in electronic equipment. It would be desirable to have batteries having higher energy density; therefore it would be desirable to have thinner separators. However, if conventional separators are simply thinned, it may not provide sufficient dendritic barrier. It would be desirable to have a separator which can be made thin, having lower ionic resistance without sacrificing barrier properties.